It's not too much like TF2 but it's the only other FPS I've played consistently in the last 8 years: Doom and Doom 2. There are thousands of custom megawads (i.e. levelsets) and hundreds of good ones on doomworld.org. You can play it multiplayer via Zandronum with a 56k or dialup connection as it requires very little bandwidth and can generally be lag-compensated without fucking up.

The base doom.wad and doom2.wad (and tnt.wad and plutonia.wad) that you need to play custom content go for like $2-4 on Steam but you can also just message me and I'll link you my dropbox that has them all--they barely break a megabyte each.